I looked for similar symptoms but could not find matching threads, so I started a new one hoping someone would be able to chime in and help out.
Background information (maybe TMI, but I figured any detail might help?):
- D90 has what I believe is the stock alternator. About 80K miles on it (i.e. "brand new"!
- Decided to play stupid and splash the crowd (which had already left....) on the last obstacle and went through a long puddle of (muddy/sandy) water.
- engine stalled after the puddle when back to idle
- after some convincing, engine restarted and could be kept running on 4, 5, 6 (???) cylinder when above 1200 rpm (or so?)
- engine dried up before getting back to camp
- stopped the truck, packed up, etc
- left camp site, drove a few miles and stopped at gas station.
- filled up
- when leaving gas station, I realized that the battery light was on and no rpm shown. Crap....
- Step 1: let wifey know that we are on our way home.
- Step 2: let wifey know that she might have to drive toward us instead of just waiting for you to at least entertain the 2x 6-year-old in the back while she helps recharging battery (or until tow truck arrives?).
- Step 3: google "how long can I drive without alternator" and keep looking until I find the answer I'm looking for ("2 hours" or "70 miles"). Of course the answer is "it depends on your battery and engine"... I was hoping to find some kind of formula or ballpark. [still interested if anyone has a number to share!
- Step 4: start praying that the alternator dries up (because it's not working only because it got wet, right?)
Anyway, while driving, I realized that the rpm came back between 2400 and 2800 rpm. Would register 0 below 2400 rpm or above 2800 rpm. This was reproducible at will.... This gave me more confidence that I'd make it home!
So, after helping a little more than usual to heat up the planet, got home, parked the truck away from everything knowing an alternator was in the cards and it'd take me a while to get to it.
Unfortunately, next day I had to move it.... And realized that the rpm were showing up across the range..... So much was hoping to troubleshoot this at a later time....
Anyway, I have not checked anything further than reading the gauges at the moment. But before I pull my voltmeter and check if the alternator puts out enough juice "now", I'd love to hear if anyone has seen the "alternator only works at a specific rpm range" before. I think I'll play safe and locate a spare alternator I'll leave in the truck so I'm not stranded when it dies (at some point it will die, right?